Several diagnostic X-ray imaging techniques have been developed in recent years which are variously called digital radiography, electronic radiography, computed radiography and digital fluoroscopy. These systems all have the common element of producing projection radiographic images in a digital form. Some of the advantages proposed for these systems are highly efficient use of dose, scatter reduction, ease of operation, noiseless data transmission, new types of image storage, flexible display capability to exploit the total range of detected information, and a potential for various forms of image manipulation such as edge enhancement, filtering, and subtraction.
One such system which achieves these advantages, and which is accordingly useful as a diagnostic modality over analog radiography systems suggested heretofore, is manufactured by American Science and Engineering, Inc., Cambridge, Mass., and is known as the Micro-Dose.RTM. system. This particular system employs, inter alia, the concept of utilizing a flying spot of X-rays to generate an image. The mechanism employed for this purpose is described generally in Stein et al U.S. Pat. No. 3,780,291 issued Dec. 18, 1973, for "Radiant Energy Imaging With Scanning Pencil Beam", reissued Sept. 2, 1975, as U.S. Pat. No. Re. 28,544, and the overall system when employed for medical diagnostic purposes is described in greater detail in the article "Digital Radiography" by P. J. Bjorkholm, M. Annis, and E. E. Frederick, Proceedings of the Society of Photo-Optical Instrumentation Engineers, Application of Optical Instrumentation in Medicine VIII, 137 (1980). In this system, the X-ray beam is shaped and positioned by mechanical collimators. More particularly, the output of a standard rotating anode X-ray tube is collimated to form a narrow fan beam of X-rays, and that fan beam is in turn intercepted by a lead-filled chopper disc having radial slits therein which are so positioned that one and only one slit always intersects the plane of the fan beam. This arrangement allows a small nearly rectangular X-ray pencil beam to pass through the disc to the subject, and causes the pencil beam to be scanned along a line as the chopper disc is rotated. The X-rays transmitted by the subject are detected by a solid state scintillator viewed by a photo tube. The output of the detector as a function of time is correlated with the chopper disc's rotational position to give the X-ray transmission as a function of position within the X-ray plane thereby to generate a one-dimensional cut through the subject. To generate the second dimension, the X-ray tube, collimator, chopper disc, and detector are translated as a unit with respect to the patient. The detector output is then digitized and sent to a computer for storage, manipulation and image creation. A single image consisting of a 512.times.480 pixel matrix is taken in about 16 seconds.
High contrast resolution, high throughput flying spot scanning systems of the type described above, and of other types to which the present invention is generally applicable, are often flux limited. Also, digital systems can be limited in spatial resolution by the number of pixels available. Both of these considerations suggest that the area scanned be as closely matched to the area of interest as possible. The system described above is capable of achieving this result only in a limited fashion and, more particularly, is so arranged that the equipment can produce field sizes of any one of three different predetermined dimensions, i.e., a large field of 15 by 20 inches, a medium field of 6 by 8 inches, and a small field of 11/2 by 2 inches. To achieve these different field sizes, the chopper disc is provided with three different sets of slits, and the chopper disc is physically moved with respect to the slit of the fan beam collimator to select that particular set of radial slits which will achieve the desired one of the field widths mentioned, while, concurrently therewith, the translational speed of the X-ray generating system and associated detector is changed to a selected one of three preset translational speeds which are factory set and which will produce the length of scan field which is preassociated with the selected scan field width during the fixed time of scanning. In short, the field size is determined laterally by the chopper wheel and fan beam geometry, and longitudinally by the translational speed of the source during the scan, but the system is so arranged that only one of three different predetermined field sizes can be selected, with the lengths and widths of these various field sizes always being in the same ratio. The position of any selected field is always fixed relative to the patient being examined and cannot be varied by manipulation of the flying spot X-ray scanning system.
Inasmuch as the dose to the patient is proportional to the area scanned, and inasmuch further as there are some radiological procedures where the area of interest is less than the normal field size and the images could be improved by increased dose, it is highly desirable to provide a system which is adapted to achieve a scan field wherein the length and width of the field can be selectively varied independently on one another, and wherein, moreover, the position of the scan field can be varied relative to the patient by controls on the equipment itself, thereby to make it possible to achieve an arbitrarily shaped and positioned field which is closely matched to the real area of interest. The ability to produce any sized rectangular field represents a considerable improvement in convenience and utility over systems suggested heretofore, and allows maximum utilization of X-ray flux and spatial resolution potential of any given digital system. The present invention is capable of achieving these highly desirable results.